1. Field of the Invention
The present invention relates to an automatic brightness adjusting method and apparatus for an image signal processor (ISP), and more particularly, to an automatic brightness adjusting method and apparatus that may adjust a brightness of an output image to an appropriate brightness so as to improve a contrast.
2. Description of the Related Art
An image signal processor (ISP) may be a processor that pre-processes an image signal input from an image sensor so that the image signal may be used in an application.
The ISP may perform image compensation, for example, color interpolation, color correction, gamma correction, color space conversion, and the like, and subsequently, may perform image compensation, for example, automatic white balancing (AWB) and automatic exposure (AE), and the like.
The AE may indicate a process that calculates a brightness of an image so as to automatically set the image to have an appropriate brightness or may indicate a block performing the process. That is, the AE may be a digital control technology that maintains the image to have an optimal brightness, irrespective of an illuminance of a light source or may be a device to which the digital control technology is applied.
The AE may sense an amount of projected light so as to control an iris, a shutter speed, and an analog gain.
Generally, the AE may calculate an average brightness of an image and may adjust a gain so that the image has the same brightness as a predetermined target brightness. A brightness of a screen is adjusted based on the AE and thus, a naturalness of the image may be enhanced.
The brightness adjustment by the AE may be insufficient to extract characteristics of an object requested by a vision system. Unlike a general camera, an image processing technology may need to be applied to the vision camera.
When a brightness of an image is intensively distributed in a predetermined range, the AE may be insufficient to clearly distinguish an outline or characteristics of an object. In this example, there is a desire for a process that broadens original intervals of brightness that are intensively distributed so as to improve a contrast of the image.
A histogram equalization (HE) may indicate a scheme that generates a conversion function based on a histogram of an input image, and may be used to improve a contrast.
The HE may generate an output image in which pixels having low brightness values and pixels having high brightness values are evenly distributed, irrespective of a histogram of brightness values of pixels of the input image. The HE may adjust an average brightness of an image to be constant. Therefore, the HE may be used instead of an AE function of the ISP.
In terms of implementation of hardware, the HE may be implemented by a simple operation and thus, an additional memory space for storing a histogram may be requested. Since the HE is simply implemented, the HE may process a relatively large image in a relatively quick period of time.
Generally, the HE may generate an output image that is equalized to be brighter, based on a histogram cumulative distribution function of the input image.
Therefore, when a brightness of the input image is intensively distributed in a relatively low brightness band or a relatively high brightness band, the output image based on the HE may have an improved contrast but may be distorted in an unnatural manner.
Examples of an algorithm that supplements the drawback of the HE may include a brightness preserving bi-histogram equalization (BBHE) algorithm, a dualistic sub-image histogram equalization (DSIHE) algorithm, a recursive mean-separate histogram (RMSHE) algorithm, and the like. The algorithms may be complex and may require a great amount of calculation. Therefore, when the algorithms are implemented in hardware, performance of an entire ISP chip may deteriorate.